The enormous increase in data traffic, largely due to the growth in Internet traffic, has spurred rapid growth in broadband communication technologies. Fiber optics, which offers the largest bandwidth of any communication system, is the medium of choice for carrying the multitude of data now being sent through networks.
A method of increasing the amount of bandwidth in a fiber optic communication system is to transmit multiple streams of information-bearing data on different wavelengths in the same physical optical transmission system. This technique, referred to as Wavelength Division Multiplexing (“WDM”), uses many wavelength channels within the same fiber to simultaneously transmit information over many streams. Thus the total bandwidth of the fiber is increased by a factor given by the number of wavelengths being used.
There are several components that make WDM systems possible, including tunable narrow band lasers, wavelength multiplexors/demultiplexors, add/drop filters, wavelength converters, and tunable receivers. All of these components are wavelength channel specific, and are used to guide the information on specific channels from the source to the destination.
Add/drop filters in WDM systems can be placed in front of an incoherent receiver and used to select a particular signal from many arriving signals. Filters can also be used to control which path though a network a signal will take.
One type of device that can be used as a filter in a WDM system is a Fabry-Perot filter, also referred to as an “Etalon”. A Fabry-Perot filter includes two highly reflective parallel mirrors that form a Fabry-Perot cavity. As input light is directed toward the first mirror; some of the light enters the cavity and some is reflected away. The light in the cavity is either transmitted through the second mirror or is reflected back toward the first mirror, which either leaves or is reflected again. Interference patterns are set up which cause the desired wavelength to be passed by the cavity but other wavelengths to destructively interfere and cancel. The desired wavelength is the resonant wavelength of the cavity.
Another known filter is a Fiber Bragg Grating (“FBG”) filter. An FBG filter is a very simple, extremely low cost, wavelength selective filter. It has a wide range of applications that both improve the quality and reduce the cost of optical networking. An FBG is a piece of single mode fiber a few centimeters in length. The grating is constructed by varying the refractive index of the core lengthwise along the fiber. Light of the specified wavelength traveling along the fiber is reflected from the grating back in the direction from which it came. Wavelengths which are not selected are passed through with little or no attenuation.
Another known filter is a thin-film resonant cavity filter (“TFF”), or a Fabry-Perot interferometer, or Etalon, where the mirrors surrounding the cavity are realized by using multiple reflective dielectric thin-film layers. This device acts as a bandpass filter, passing through a particular wavelength and reflecting all the other wavelengths. The wavelength that is passed through is determined by the cavity length.
Another type of filter is a Mach-Zehnder interferometer (“MZI”), which is an interferometric device that makes use of two interfering paths of different lengths to resolve different wavelengths. Mach-Zehnder interferometers are typically constructed of integrated optics and consist of two 3-dB directional couplers interconnected through two paths of different lengths.
Still another type of filter is an arrayed waveguide grating (“AWG”), which uses the same principle as the Mach-Zender interferometer. An AWG consists of two multiple-input, multiple-output couplers that are connected together by an array of waveguides, each having different lengths. The AWG has multiple copies of the same signal shifted in phase by different amounts added together, while the MZI has only two.
In general, one problem with many of the above-described filters is a lack of tunability. Specifically, in many of the above-described filters the wavelength channels that are selected for transmission must be selected during manufacturing and cannot be changed or tuned when placed in a network. Many of these filters also suffer from being large in size and therefore difficult to implement.
Based on the foregoing, there is a need for an improved optical add/drop filter that can be used in a WDM system.